This invention relates to optical waveguides and more particularly to a control of input and output of light in a waveguide system, and/or to the modification of propagation characteristics of optically guided light.
The propagation of light in optical waveguides is of current interest because these systems can perform many data-processing and communication functions. Integrated optical technology may be envisioned to combine a system that is capable of modulating, switching, and detecting with optical microcircuitry. A microoptical system is desirable since it is rigid, free from environmental effects, and capable of handling greater volumes of information than traditional electronic systems.
Optical waveguides are well known in the art and it is well known in the art to use coherent light with such waveguides. An optical waveguide consists of a region in which the index of refraction is greater than the index of refraction of the surrounding medium, top and bottom. Such has been set forth in an article "Light Waves in Thin Films and Integrated Optics", P. K. Tien, Applied Optics, 10, pages 2395, 1971. The methods for fabricating substrates of given fixed indices of refraction for any given index between 1.3 and 3.0 are well known. Coherent light propagating along the waveguide region will suffer total internal reflection at the boundries between the waveguide region and the surrounding medium. If the angle of incidence .THETA. is greater than the critical angle .THETA..sub.c, that is, .THETA.&gt; .THETA..sub.c = Sin .sup..sup.-1 (N.sub.0,.sub.2 /N.sub.1) where the index of refraction of the upper layer is N.sub.0, the waveguide region N.sub.1 and the lower layer N.sub.2. The coherent light traveling in the waveguide is trapped in the waveguide region by being totally reflected from the upper and lower interfaces between the waveguide region and the upper and lower medium. The nature of the coherent light trapped in the waveguide is subject to the boundry conditions, i.e., index of refraction of the substrate, index of refraction of the media and angle of insertion of the light. Depending on the difference in the indices of refraction (media) and substrates, different discrete angles of insertion can be allowed, since the solution of the boundry condition problem provides for only Eigen valued solutions. Thus, only a discrete number of modes can be supported in the waveguide. The greater the difference in the index of refraction between the media and the substrate, the larger the number of discrete modes that can be supported.
Now considering those properties of waveguides which lead to low-loss propagation, to techniques of input and output couplings, and to optical switching and modulation, there is considerable discussion in the literature concerning various topological shapes of optical waveguides and their characteristics. Waveguides may be flat slabs, rectangular, cylindrical, or any other shape. Metal-clad dielectrics may not be used in optical waveguides due to the high losses. When light is reflected off a metal surface, the energy losses depend on the metal, the wavelength of the light, surface condition, polarization, and angle of incidence, and losses are typically about 1 or 2% per reflection. Hence, metal-clad waveguides, because of the number of reflections per centimeter, are extremely lossy in the optical region. In the dielectric-clad waveguide, losses are due to absorption in the dielectrics and scattering losses. If the cladding has no absorption at the optical wavelength being propagated, then no energy is absorbed from the evanescent wave (extending into the cladding) and the waveguide suffers only from scattering losses.
To couple coherent light into and out of a waveguide, it is necessary to change the boundary conditions. This may be done in several ways, by use of a prism, an optical grating, or tapered edge. In the prism coupler, a prism is brought within a few optical wavelengths of a waveguide surface and frustrates total internal reflection at the surface. Frustrated total internal reflection or evanescent-field coupling, occurs because the boundary conditions of a waveguide are modified by the presence of the prism. A second type of coupler, the grating coupler, comprises a periodic structure in contact with the waveguide boundary. This structure permits momentum matching between a guided optical wave and a wave propagating in the cladding medium and thus provides coupling between the two waves. Another coupler includes a tapered edge or edge fired coupler.
To construct active devices, the index of the waveguide media, or the determining boundary conditions, must be actively controlled by some external parameter. Once this is achieved, the construction of systems that will act as modulators, deflectors, and switches is possible.
This invention makes use of a viscous fluid such as liquid crystals either cholesteric, smectic or nematic. Liquid crystals are substances which upon being melted instead of becoming a clear liquid, they pass through a turbid fluid state which is termed the mesomorphic or liquid crystal state. Liquid crystals useful in this case are well known in prior art and have been set forth in U.S. Pat. No. 3,322,485, for example. Also the following liquid crystals are useful: Nematic -4' Cyano 4' Pentyl Biphenyl; Smectic 4 Cyano 4' Octyl biphenyl; Cholesteric; Cholesteryl, Nononoate, Deconoate, Chloride. Liquid crystal properties have been set forth in "Prospectus for the Development of Liquid Crystal Waveguides", NRL Report 7507, by Joel M. Schnur and Thomas G. Giallorenzi, published by Naval Research Laboratory, Washington, D.C. 20375.